A. Radiation-Balanced Lasers and Laser Cooling
Waste heat generation is a generic problem in laser systems. Reduction of thermal loading is of crucial importance in high-power and cryogenic laser applications. The process of excitation and stimulated emission in conventional solid-state/semiconductor/organic lasers results in heat generation in the lasing medium. This is always the case because of the Stokes energy shift between the higher-energy pump photons (for optical pumping) or injected carriers (for electrical current injection) and the lower-energy laser output photons, often called the quantum defect. This means conventional solid-state/semiconductor/organic lasers are always exothermic. In 1999, a concept of a solid-state bulk laser was introduced without internal heat generation, nowadays called a radiation-balanced or a thermal laser. The use of radiation cooling by anti-Stokes fluorescence within the laser medium has been suggested to balance the heat generated by the Stokes-shifted stimulated emission. A laser cooling cycle in a solid 100 is illustrated in FIG. 1. The upper 110 and lower 120 electronic levels (manifolds) are split into many closely spaced sublevels (energy bands in case of semiconductor materials). Pump photons at the long wavelength tail of the absorption spectrum with the energy hvp excite the low-energy electronic transitions from the ground state to the excited state. The excited ions in a host matrix (or carriers in a semiconductor material) absorb phonons during the thermalization process and reach quasi-equilibrium with the lattice. Fluorescence then follows with a mean photon energy hvf higher than that of the absorbed photon, thus removing energy from the sample. For electrical current injection in semiconductor injection lasers, the pump energy is given by e U, where U is the voltage applied to the p-n junction.
The essential condition for achieving cooling in solids is availability of a high quantum efficiency anti-Stokes transition and low non-radiative recombination rate. It is equally important that anti-Stokes spontaneous emission escapes the material without trapping and reabsorption, which would cause re-heating of the sample. These requirements can be satisfied for rare-earth ions in hosts with low phonon energy and low-index of the host material, such as fluoride or chloride glasses and crystals. In semiconductors, however, the problems with the realization of laser cooling include relatively high nonradiative recombination rate, low extraction efficiency of spontaneous emission due to the total internal reflection, and the reabsorption effect. Laser cooling of semiconductors has been attempted for decades in III-V semiconductor quantum wells without success. Significant breakthroughs have been recently reported by using II-VI, rather than III-V, nanomaterials. In particular, laser cooling by 40 K has been observed in II-VI CdS nanoribbons and by 30 K in CdS nanobelts. Very intense phonon-assisted anti-Stokes photoluminescence and even lasing has recently been reported in ZnTe nanoribbons. The net laser cooling in those II-VI materials was attributed to strong coupling between excitons and longitudinal optical phonons (LOPs) that allowed the resonant annihilation of multiple LOPs in the luminescence up-conversion processes, high external quantum efficiency, and negligible background absorption. These II-VI materials are very promising for development of self-cooled and radiation-balanced semiconductor lasers, where anti-Stokes emission would mitigate the heating effect.
As mentioned above, a significant challenge that needs to be addressed in radiation-balanced lasers is the photoluminescence trapping and the consequent photon recycling. Effective strategies to reduce photon trapping are needed to assist laser cooling. Photons get trapped inside a material due to the total internal reflection. The escape cone for a boundary between a bulk material with index n and air is equal to 2n24π steradians, which leads to an extraction efficiency of ˜½n2. The index and the size of the structure determines the amount of the power radiated out of the material—the higher the index and size of the material the higher the localization of the optical mode inside the material. This simple formula predicts a very low extraction efficiency of ˜5.5% for semiconductors with the refractive index of 3, not usually sufficient for laser cooling purposes. Moreover, in cryogenic environments, the problem of photon trapping is only partially solved with the energy extracted out of the sample. Unless it leaves the cryogenic chamber completely, the isotropic spontaneous emission will eventually get reabsorbed and will heat the cryogenic environment. Therefore, it is very desirable to find a strategy for directional extraction of spontaneous emission out of the laser device and outside of the cryogenic chamber.
B. Thermal Considerations in High-Power Semiconductor Lasers
High-power semiconductor lasers are in high demand in industrial, medical, military, communication, recordable optical data storage, and other fields. The main factor limiting the maximum power of a high-power semiconductor laser in continuous wave (CW) operation is self-heating at high drive currents determined by electrical to optical power conversion efficiency and the thermal load that the laser assembly can dissipate.
Self-heating is detrimental in several ways leading, for example, to thermal rollover and catastrophic optical damage in semiconductor lasers. At higher internal temperature, active region gain drops significantly, whereas carrier leakage from the active region and Auger recombination rate increases. Thus, the laser has a higher threshold and lower slope efficiency, that is lower power efficiency. To reach a certain power level at high temperature, the pump current should be much higher than that at room temperature. Thermal rollover thus occurs at high drive currents, with output power peaking at a particular point and then reducing with additional increase in current. The catastrophic optical damage in semiconductor lasers is a failure mode described as thermal runaway process in which a local temperature increase plays an important role. The locally increased temperature causes shrinkage of the active region energy bandgap with a corresponding enhancement of the optical absorption and eventual melting of the active region semiconductor material in a feedback process.
The two key countermeasures against self-heating in high-power semiconductor lasers are to maximize their power conversion efficiency (PCE) and minimize thermal resistance. To date, the highest values of PCE are demonstrated in GaAs-based broad-area lasers. The highest reported PCE at heat sink temperature >0° C. is 76% for devices at 975 nm, and PCE over 70% has been reported by several research groups for single emitters and laser bars in the 900-1000 nm wavelength range. However, the demonstrated high peak efficiencies typically occur at relatively low power per emitter, in the 2-5 W range for single emitters with stripe width ˜90 μm, which is insufficient for many applications. Efficiency increase of ˜10% relative to conventional designs has been demonstrated at high powers in semiconductor lasers of extreme double asymmetric design. The thermal resistance is dependent on the laser chip/bar geometry, such as the pumped area to total chip/bar area ratio (fill factor) and the cavity length. A larger thermal footprint enables a lower operating temperature at a given heat load.
The major characteristics of high-power lasers are strongly affected by the quality of the package designed for efficient heat transfer from the junction and by the cooling mechanisms used to remove the heat. Depending on the thermal power density, two different types of heat sinks are used: active and conductive. For CW and long-pulse operational mode, active cooling is necessary. The active heat sinks can further be subdivided into liquid-cooled micro- or macro-channel heat sinks, liquid-impingement jets, and evaporative sprays. Some high-power laser applications, however, require that high-power lasers operate in a high temperature environment without any active cooling. Therefore, an alternative strategy for cooling high-power semiconductor lasers is highly desirable in support of passive conductive cooling for such applications.
C. Strongly Injection-Locked Unidirectional Whistle-Geometry Microring Lasers
To control directionality of lasing in ring resonators, a novel whistle-geometry ring laser (WRL) structure 200 is shown in FIG. 2 and described in U.S. Pat. No. 8,009,712, the teachings of which are incorporated herein by reference. The WRL structure is particularly attractive when strong injection of external light into the ring resonator is desirable, for example in the case of high-speed semiconductor lasers. As illustrated in FIG. 3, the WRL geometry allows for strong coupling of a single-frequency master laser output into the slave laser. The advantage of the injection-locking scheme illustrated in FIG. 3 for ultra-high-speed modulation was confirmed in numerical modeling.
D. Spontaneous Emission Control in Dielectric-Waveguide-Based Laser Cavities
For the full directional control of the spontaneous emission in a cavity to be realized, all the spontaneous emission events must couple spatially into the guided modes. In other words, a spontaneous emission pattern that emits solely into the guided modes, which requires minimization of spontaneous emission into the radiation modes is desired. A typical situation in dielectric-waveguide-based resonators, however, is just the opposite. The guided modes are supported by the total internal reflection and the active region waveguide is designed to support only fundamental transverse guided mode. With the low refractive index contrast between the active region waveguide core and the surrounding cladding layers, most of the spontaneous emission escapes from the active region layer and gets trapped inside the laser chip due to small escape cone for the interface between the bulk material with the refractive index n 3 and air with n=1.
E. Spontaneous Emission Control by Photonic Crystals
In contrast to dielectric-waveguide-based resonators, the resonant modes in photonic crystal (PhC) resonators (PCRs) are supported by the photonic bandgap (PBG) effect, which is much more efficient for optical confinement. For example, PhC structures have the potential to achieve high-Q, low-loss resonators in ultra-compact cavities several times smaller than the minimum-possible-size dielectric-waveguide-based resonators. What is of even greater importance is the unique capability of PhCs to control spontaneous emission due to PBG effect, not attainable in conventional dielectric-waveguide-based cavities. Spontaneous-emission control by 2D PhCs has been demonstrated. It has been shown that the overall spontaneous emission rate can be substantially reduced by the 2D PBG, while the light-emission efficiency for a direction where the 2D PBG is not present, can be significantly enhanced. This result clearly demonstrates that when spontaneous emission is inhibited by the 2D PBG effect in a certain direction (and thus the excited carriers are forbidden to recombine by emitting spontaneous photons in that direction), the carriers are eventually used by emitting spontaneous photons in other directions. This effect was referred to as “the inhibition of spontaneous emission and the redistribution of the saved energy.” Theoretically, a near-unity efficiency of a quantum dot emitter coupling to a PhC waveguide has been predicted. In general, photon density of states (DoS) modification allowed by PhCs can be used to boost the efficiency of the optical refrigeration in two ways. First, a fluorescence peak can be blue-shifted by suppressing the photon DoS in a spectral range below the pump frequency vp (Stokes component of spontaneous emission), thus increasing vf. Additionally, an increase of the radiative rate at the blue-shifted emission wavelength could increase the external quantum efficiency ηext.